I feel for you man, I really do. I'm pretty much in the same boat as you. Talking helps a lot. Try and talk through why you are feeling the way you do. If you talk enough, you start getting down through the layers, and hopefully to the base reason.
I wish I had a good fix for you, but I'm still feeling a lot of the same things you are. A lot of what she is feeling is called New Relationship Energy (NRE). It's a hard stage to go through, because of the fact that it's somebody else that is making her happy. For Jen and I, it brought a very deep level of communication, and we have been very close since then.
Have you met the guy yet? I would suggest maybe meeting him first before they start a relationship. That way at least you will know who it is and everything.